\newpage
\section{Evaporation}

\subsection{Humidity} % (fold)
\label{sub:Humidity}
\subsubsection{Meaning of $t_a$, $t_w$ and $t_d$} % (fold)
\begin{itemize}
  \item[\textbf{$t_a$}] stands for \emph{dew point temperature} and is the temperature at which a parcel of air would condense, also called the saturation point.
  \item[\textbf{$t_w$}] means the \emph{wet temperature} and is the temperature of the wet thermometer.
  \item[\textbf{$t_d$}] is the \emph{actual temperature} measured by the dry thermometer.
\end{itemize}
% subsubsection Test (end)
\subsubsection{Meaning of measured temperatures} % (fold)
25.0 \celsius \, is the actual (dry bulb) temperature $t_a$. \\
17.5 \celsius \, is the wet bulb temperature $t_a$.

\subsubsection{Saturation and actual vapour pressure} % (fold)

The vapour pressure $e(t)$ can be calculated with the following relation:

\begin{equation}
 0.61{e^{\frac{{17.3t}}{{237 + t}}}}
\end{equation}
\\
It follows that the saturation vapour pressure for 17.5 \celsius \, equals 2.0 kPa. \\
\\
Using
\begin{equation}
 e_a(t_a)-e_s(t_w) = -0.066(t_a-t_w)
\end{equation}
\\
the actual vapour pressure $e_a$ can be found and equals 2.68 kPa.

\subsubsection{Psychrometer constant} % (fold)
The psychrometer constant indicates the relation between the actual and saturated vapour pressure. It's unit is in kPa/\celsius.
% subsubsection Psychometer constant (end)

\subsubsection{Relative humidity} % (fold)
\label{ssub:Relative humidity}
The relative humiditiy is the ratio between the actual vapour pressure $e_a$ and the saturated vapour pressure $e_s$ and can be determined with: \\
\begin{equation}
  h=\frac{e_a(t_a)}{e_s(t_a)}
\end{equation}
\\
The relative humidity in this problem equals 84\%
